Cruise Control

This weeks technical article is about cruise control. This device is both useful and dangerous: useful because it can save you money when driving in a speed camera infested area but dangerous because drivers activate it on freeways and then drive in any lane at one speed – even if they are in the blind spot of a truck.

Cruise Control is an idea that has jumped from stationary engines to cars. James Watt, the inventor of the steam engine, designed the first “cruise control” to manage his engine and in the early years of the car industry, the US Peerless car company – not the same company that morphed into Gordon-Keeble – used a centrifugal governor on their models. This kept the RPM at a constant rate.

Like power steering, it was the Chrysler Imperial, or specifically the 1958 Imperial that introduced the modern style Cruise Control to the market. A few years earlier, Chrysler had spun off the Imperial model as a brand in its own right, but still using shared technology with the parent company.

Cruise Control works with the aid of sensors on the prop shaft and also on the brake and clutch pedals. If one of the pedals are touched, the system switches off. The driver has to manually accelerate to the desired speed and then when the Cruise Control is activated it will maintain that speed.

The controls are connected to a vacuum actuator that controls the throttle and is itself controlled by a CPU and electronic gadgetry that ensures the right fuel/air mix to provide the desired speed. The electronics are also waiting to get feedback from other places, like the pedals, in case the car needs to decelerate and from speed sensors to ensure that it knows what the current situation is.

There are three main factors that the electronics deal with, the proportional control, the integral control and the derivative control:
– The proportional control is the amount that the throttle is open. As the car reaches its desired speed it will accelerate more slowly as the throttle is closed to achieve the desired speed.
– The integral control is based on distance, so the CPU is working out the distance travelled at the current speed and the distance that would have been travelled at the desired speed. Time is then factored in and this helps the car deal with gradients and other factors that could change the speed without the driver’s input.
– The final factor is the derivative control. The derivative of speed is acceleration and this comes into effect again when gradients are encountered because this control is sensing the speed of acceleration.
So when a car travels up a hill, these three controls combine their data to provide the optimal throttle opening to keep the speed constant.

In the future we will see more cars fitted with adaptive cruise control. This system also factors in the distance between the car and others in front, keeping a safe gap in case of emergencies. We are now in an era of seeing a lot of the functions of cars being automated so that the driver does not have to think too much. Personally I think that is why there are accidents because drivers aren’t paying attention because the driving is now done for them.

Update – 14th February 2017
I had a great email from Vince, an engineer, who made some real world observations. In his experience he has seen systems that manage the proportional and integral controls, however he pointed out that the derivative control is redundant on most vehicles as this comes into effect when there is a sudden change in speed – something that rarely happens with road cars in normal operation.

First published: 12th February 2017. Last updated: 19th May 2018.